The ability to read, and manipulate Perl (the language) programmatically other than with perl (the application) was one that caused difficulty for a long time.

The cause of this problem was Perl's complex and dynamic grammar. Although there is typically not a huge diversity in the grammar of most Perl code, certain issues cause large problems when it comes to parsing.

Indeed, quite early in Perl's history Tom Christiansen introduced the Perl community to the quote "Nothing but perl can parse Perl", or as it is more often stated now as a truism:

"Only perl can parse Perl"

One example of the sorts of things the prevent Perl being easily parsed are function signatures, as demonstrated by the following.

The first line above can be interpreted in two different ways, depending on whether the &dothis function is expecting one argument, or two, or several.

A "code parser" (something that parses for the purpose of execution) such as perl needs information that is not found in the immediate vicinity of the statement being parsed.

The information might not just be elsewhere in the file, it might not even be in the same file at all. It might also not be able to determine this information without the prior execution of a BEGIN {} block, or the loading and execution of one or more external modules. Or worse the &dothis function may not even have been written yet.

When parsing Perl as code, you must also execute it

Even perl itself never really fully understands the structure of the source code after and indeed as it processes it, and in that sense doesn't "parse" Perl source into anything remotely like a structured document. This makes it of no real use for any task that needs to treat the source code as a document, and do so reliably and robustly.

For more information on why it is impossible to parse perl, see Randal Schwartz's seminal response to the question of "Why can't you parse Perl".

The purpose of PPI is not to parse Perl Code, but to parse Perl Documents. By treating the problem this way, we are able to parse a single file containing Perl source code "isolated" from any other resources, such as libraries upon which the code may depend, and without needing to run an instance of perl alongside or inside the parser.

Historically, using an embedded perl parser was widely considered to be the most likely avenue for finding a solution to Parse::Perl. It was investigated from time to time and attempts have generally failed or suffered from sufficiently bad corner cases that they were abandoned.

PPI is an acronym for the longer original module name Parse::Perl::Isolated. And in the spirit or the silly acronym games played by certain unnamed Open Source projects you may have hurd of, it also a reverse backronym of "I Parse Perl".

Of course, I could just be lying and have just made that second bit up 10 minutes before the release of PPI 1.000. Besides, all the cool Perl packages have TLAs (Three Letter Acronyms). It's a rule or something.

Why don't you just think of it as the Perl Parsing Interface for simplicity.

The original name was shortened to prevent the author (and you the users) from contracting RSI by having to type crazy things like Parse::Perl::Isolated::Token::QuoteLike::Backtick 100 times a day.

In acknowledgment that someone may some day come up with a valid solution for the grammar problem it was decided at the commencement of the project to leave the Parse::Perl namespace free for any such effort.

Since that time I've been able to prove to my own satisfaction that it is truly impossible to accurately parse Perl as both code and document at once. For the academics, parsing Perl suffers from the "Halting Problem".

With this in mind Parse::Perl has now been co-opted as the title for the SourceForge project that publishes PPI and a large collection of other applications and modules related to the (document) parsing of Perl source code.

Make structural, syntax, or other changes to code in an automated manner, either independently or in assistance to an editor. This sort of task list includes backporting, forward porting, partial evaluation, "improving" code, or whatever. All the sort of things you'd want from a Perl::Editor.

Change the layout of code without changing its meaning. This includes techniques such as tidying (like perltidy), obfuscation, compressing and "squishing", or to implement formatting preferences or policies.

This includes methods of improving the presentation of code, without changing the content of the code. Modify, improve, syntax colour etc the presentation of a Perl document. Generating "IntelliText"-like functions.

If we treat this as a baseline for the sort of things we are going to have to build on top of Perl, then it becomes possible to identify a standard for how good a Perl parser needs to be.

PPI seeks to be good enough to achieve all of the above tasks, or to provide a sufficiently good API on which to allow others to implement modules in these and related areas.

However, there are going to be limits to this process. Because PPI cannot adapt to changing grammars, any code written using source filters should not be assumed to be parsable.

At one extreme, this includes anything munged by Acme::Bleach, as well as (arguably) more common cases like Switch. We do not pretend to be able to always parse code using these modules, although as long as it still follows a format that looks like Perl syntax, it may be possible to extend the lexer to handle them.

The ability to extend PPI to handle lexical additions to the language is on the drawing board to be done some time post-1.0

The goal for success was originally to be able to successfully parse 99% of all Perl documents contained in CPAN. This means the entire file in each case.

PPI has succeeded in this goal far beyond the expectations of even the author. At time of writing there are only 28 non-Acme Perl modules in CPAN that PPI is incapable of parsing. Most of these are so badly broken they do not compile as Perl code anyway.

So unless you are actively going out of your way to break PPI, you should expect that it will handle your code just fine.

PPI provides partial support for internationalisation and localisation.

Specifically, it allows the use characters from the Latin-1 character set to be used in quotes, comments, and POD. Primarily, this covers languages from Europe and South America.

PPI does not currently provide support for Unicode, although there is an initial implementation available in a development branch from CVS.

If you need Unicode support, and would like to help stress test the Unicode support so we can move it to the main branch and enable it in the main release should contact the author. (contact details below)

When PPI parses a file it builds everything into the model, including whitespace. This is needed in order to make the Document fully "Round Trip" safe.

The general concept behind a "Round Trip" parser is that it knows what it is parsing is somewhat uncertain, and so expects to get things wrong from time to time. In the cases where it parses code wrongly the tree will serialize back out to the same string of code that was read in, repairing the parser's mistake as it heads back out to the file.

The end result is that if you parse in a file and serialize it back out without changing the tree, you are guaranteed to get the same file you started with. PPI does this correctly and reliably for 100% of all known cases.

What goes in, will come out. Every time.

The one minor exception at this time is that if the newlines for your file are wrong (meaning not matching the platform newline format), PPI will localise them for you. (It isn't to be convenient, supporting arbitrary newlines would make some of the code more complicated)

Better control of the newline type is on the wish list though, and anyone wanting to help out is encouraged to contact the author.

PPI is built upon two primary "parsing" components, PPI::Tokenizer and PPI::Lexer, and a large tree of about 50 classes which implement the various the Perl Document Object Model (PDOM).

The PDOM is conceptually similar in style and intent to the regular DOM or other code Abstract Syntax Trees (ASTs), but contains some differences to handle perl-specific cases, and to assist in treating the code as a document. Please note that it is not an implementation of the official Document Object Model specification, only somewhat similar to it.

On top of the Tokenizer, Lexer and the classes of the PDOM, sit a number of classes intended to make life a little easier when dealing with PDOM trees.

Both the major parsing components were hand-coded from scratch with only plain Perl code and a few small utility modules. There are no grammar or patterns mini-languages, no YACC or LEX style tools and only a small number of regular expressions.

This is primarily because of the sheer volume of accumulated cruft that exists in Perl. Not even perl itself is capable of parsing Perl documents (remember, it just parses and executes it as code).

As a result, PPI needed to be cruftier than perl itself. Feel free to shudder at this point, and hope you never have to understand the Tokenizer codebase. Speaking of which...

The Tokenizer takes source code and converts it into a series of tokens. It does this using a slow but thorough character by character manual process, rather than using a pattern system or complex regexes.

Or at least it does so conceptually. If you were to actually trace the code you would find it's not truly character by character due to a number of regexps and optimisations throughout the code. This lets the Tokenizer "skip ahead" when it can find shortcuts, so it tends to jump around a line a bit wildly at times.

In practice, the number of times the Tokenizer will actually move the character cursor itself is only about 5% - 10% higher than the number of tokens contained in the file. This makes it about as optimal as it can be made without implementing it in something other than Perl.

In 2001 when PPI was started, this structure made PPI quite slow, and not really suitable for interactive tasks. This situation has improved greatly with multi-gigahertz processors, but can still be painful when working with very large files.

The target parsing rate for PPI is about 5000 lines per gigacycle. It is currently believed to be at about 1500, and main avenue for making it to the target speed has now become PPI::XS, a drop-in XS accelerator for PPI.

Since PPI::XS has only just gotten off the ground and is currently only at proof-of-concept stage, this may take a little while. Anyone interested in helping out with PPI::XS is highly encouraged to contact the author. In fact, the design of PPI::XS means it's possible to port one function at a time safely and reliably. So every little bit will help.

The Lexer takes a token stream, and converts it to a lexical tree. Because we are parsing Perl documents this includes whitespace, comments, and all number of weird things that have no relevance when code is actually executed.

Each Document will contain a number of Statements, Structures and Tokens.

A PPI::Statement is any series of Tokens and Structures that are treated as a single contiguous statement by perl itself. You should note that a Statement is as close as PPI can get to "parsing" the code in the sense that perl-itself parses Perl code when it is building the op-tree.

Because of the isolation and Perl's syntax, it is provably impossible for PPI to accurately determine precedence of operators or which tokens are implicit arguments to a sub call.

So rather than lead you on with a bad guess that has a strong chance of being wrong, PPI does not attempt to determine precedence or sub parameters at all.

At a fundamental level, it only knows that this series of elements represents a single Statement as perl sees it, but it can do so with enough certainty that it can be trusted.

However, for specific Statement types the PDOM is able to derive additional useful information about their meaning. For the best, most useful, and most heavily used example, see PPI::Statement::Include.

A PPI::Structure is any series of tokens contained within matching braces. This includes code blocks, conditions, function argument braces, anonymous array and hash constructors, lists, scoping braces and all other syntactic structures represented by a matching pair of braces, including (although it may not seem obvious at first) <READLINE> braces.

Each Structure contains none, one, or many Tokens and Structures (the rules for which vary for the different Structure subclasses)

Under the PDOM structure rules, a Statement can never directly contain another child Statement, a Structure can never directly contain another child Structure, and a Document can never contain another Document anywhere in the tree.

As you can see, the tree can get fairly deep at time, especially when every isolated token in a bracket becomes its own statement. This is needed to allow anything inside the tree the ability to grow. It also makes the search and analysis algorithms much more flexible.

Because of the depth and complexity of PDOM trees, a vast number of very easy to use methods have been added wherever possible to help people working with PDOM trees do normal tasks relatively quickly and efficiently.

The core PPI distribution is pure Perl and has been kept as tight as possible and with as few dependencies as possible.

It should download and install normally on any platform from within the CPAN and CPANPLUS applications, or directly using the distribution tarball. If installing by hand, you may need to install a few small utility modules first. The exact ones will depend on your version of perl.

There are no special install instructions for PPI, and the normal Perl Makefile.PL, make, make test, make install instructions apply.

You are recommended to use the PPIx:: namespace for PPI-specific modifications or prototypes thereof, or Perl:: for modules which provide a general Perl language-related functions.

If what you wish to implement looks like it fits into PPIx:: namespace, you should consider contacting the Parse::Perl mailing list (detailed on the SourceForge site) first, as what you want may already be in progress, or you may wish to consider joining the team and doing it within the Parse::Perl project itself.

Write access to the repository is made available automatically to any published CPAN author, and to most other volunteers on request.

If you are able to submit your bug report in the form of new (failing) unit tests, or can apply your fix directly instead of submitting a patch, you are strongly encouraged to do so, as the author currently maintains over 100 modules and it can take some time to deal with non-"Critical" bug reports or patches.

This will also guarantee that your issue will be addressed in the next release of the module.

For large changes though, please consider creating a branch so that they can be properly reviewed and trialed before being applied to the trunk.

If you cannot provide a direct test or fix, or don't have time to do so, then regular bug reports are still accepted and appreciated via the GitHub bug tracker.

A huge thank you to Phase N Australia (http://phase-n.com/) for permitting the original open sourcing and release of this distribution from what was originally several thousand hours of commercial work.

And finally, thanks to those brave ( and foolish :) ) souls willing to dive in and use, test drive and provide feedback on PPI before version 1.000, in some cases before it made it to beta quality, and still did extremely distasteful things (like eating 50 meg of RAM a second).

I owe you all a beer. Corner me somewhere and collect at your convenience. If I missed someone who wasn't in my email history, thank you too :)

And to single one person out, thanks go to Randal Schwartz who spent a great number of hours in IRC over a critical 6 month period explaining why Perl is impossibly unparsable and constantly shoving evil and ugly corner cases in my face. He remained a tireless devil's advocate, and without his support this project genuinely could never have been completed.

So for my schooling in the Deep Magiks, you have my deepest gratitude Randal.